For all-wheel drive motor vehicles, a distinction is made between an engageable all-wheel drive and a permanent all-wheel drive. In the case of a permanent all-wheel drive, the drive torque produced in the drive assembly is distributed via a mechanical inter-axle differential, such as, for example, a Torsen differential, essentially uniformly to the front axle and to the rear axle. Torque or speed differences are compensated for via the inter-axle differential between the two vehicle axles. In contrast to this, for an engageable all-wheel drive, the inter-axle differential is replaced by a rigid center clutch. By means of the center clutch, it is possible, as needed, to engage the rear axle with the drive train or to decouple it from the drive train. In such an engageable all-wheel drive—in contrast to the above permanent all-wheel drive with an inter-axle differential—there is no speed or torque compensation between the vehicle axles.
Known from DE 10 2012 020 908 A1 is a generic drive device for an all-wheel drive, two-track motor vehicle. Arranged in the drive train of the motor vehicle is a drive assembly constructed from an internal combustion engine with a downstream manual transmission, via which, in driving operation, the front axle is permanently driven. The transmission output shaft extends up to a center clutch, which is in drive connection with the rear axle via a Cardan shaft. In the closed state, the center clutch is subjected to a coupling torque. In order to ensure the absence of slippage, the center clutch can be operated in the closed state with excess contact pressure. In this case, the rear axle is engaged rigidly with the drive train. In the open or disengaged state of the center clutch, the rear axle is decoupled from the drive train.
In the above rigid all-wheel drive, the engine torque delivered by the drive assembly in driving operation is distributed in accordance with the axle friction coefficient (which acts between the vehicle wheels and the roadway) at the front axle and with the axle friction coefficient at the rear axle. By way of example, for equal axle friction coefficients, the wheel torque that can be taken up at the front axle and at the rear axle is, respectively, 50% of the total wheel torque delivered by the engine.
In a driving situation with engaged all-wheel drive as well as with the front axle on asphalt and with the rear axle on ice, the following constellation results: in this case, the front axle provides a greater ability to take up torque than the rear axle. This means that a greater wheel torque can be taken up at the front axle than at the rear axle with a smaller axle friction coefficient. This can lead to the fact that the wheel torque applied at the front axle exceeds a critical overload threshold value. This results in the danger of an axle overload, in particular an overload of the bevel gear of the front-axle differential.
For protection against such an axle overload, a component protection function can be created in a motor vehicle with purely front-wheel drive, for which the maximum allowed engine torque is calculated from an addition of an overload threshold value, which is predetermined by design and is deposited in the control instrument and for which, when it is reached, there exists the danger of a component damage at the front axle, with the moment of inertia of the engine. The moment of inertia of the engine is needed for acceleration of the engine and therefore does not place a load on the front axle.
Although, in this way, a protection of the components of the front axle against overload is ensured, the engine torque limitation results in limitations in regard to the engine performance as well as a time loss during the starting operation and reduced hill climbing and starting abilities on slopes.